Tal Hoffman

Introduction to CRIU and Live migration

In this blog post I will try and explain what CRIU is and how it works, what Live Migration is, and how those two are related.

So what the heck is CRIU?

Checkpoint-Restore in Userspace (or CRIU) is a really (really) cool open-source project started by virtualization software company — Virtuozoo — also known for being the creator of OpenVZ.

What CRIU does is letting you freeze running Linux processes and checkpoint their state to the disk as a collection of files. Those files can later be used to restore a process right from the point it’d been freezed, multiple times, on any other CRIU-supported Linux machine!

Among its many usage scenarios, CRIU can be used for slow-boot service speed up, remote debugging, snapshots, process duplication, and for what is our main topic today — live migrations.

Take a look at https://criu.org/Usage_scenarios for more

CRIU is now integrated as part of Podman, Docker (experimental), OpenVZ, LXC/LXD, and can also be used independently.

The mechanics (in a nutshell)

Let’s take a little dive into the internals of how this magic happens.


The first step when checkpointing a process is walking recursively through its tree, and freezing it so it will not change its state while CRIU needs to dump it.

CRIU supports two different methods for freezing the state of the process and its sub tasks.

By default, CRIU makes use of ptrace to stop the process. For those not familiar with ptrace:

The ptrace() system call provides a means by which one process (the "tracer") may observe and control the execution of another process (the "tracee"), and examine and change the tracee's memory and registers. It is primarily used to implement breakpoint debugging and system call tracing.

See https://man7.org/linux/man-pages/man2/ptrace.2.html.

In this method, CRIU first lists and goes through the relevant /proc/$pid entries. Thread ids are collected through /proc/$pid/task, whereas sub-processes are recursively collected through reading /proc/$pid/task/$tid/children files.

Each task it encounters — parent process itself, sub-processes, and threads — is being attached to CRIU’s tracer process by dispatching a PTRACE_SEIZE request, after which a PTRACE_INTERRUPT request is also dispatched in order to stop that task.

The second method for freezing the process tree is using Linux’s Cgroup Freezer — available through CRIU’s --freeze-cgroup flag. Cgroup Freezer is a subsystem supported by the Linux Kernel which lets us start and stop a set of tasks (i.e, processes and threads), by defining a control group.

Here is a simple example of usage:

# Create a cgroup freezer directory
mkdir /sys/fs/cgroup/freezer

# Mount directory against a cgroup filesystem of type 'freezer'  
mount -t cgroup -ofreezer freezer /sys/fs/cgroup/freezer

# Create a child cgroup directory
mkdir /sys/fs/cgroup/freezer/whatever

# Put a task into this cgroup
echo $some_pid > /sys/fs/cgroup/freezer/whatever/tasks

# Freeze cgroup
echo FROZEN > /sys/fs/cgroup/freezer/whatever/freezer.state

# cat /sys/fs/cgroup/freezer/whatever/freezer.state

# cat /sys/fs/cgroup/freezer/whatever/freezer.state

# Thaw (unfreeze) it
echo THAWED > /sys/fs/cgroup/freezer/whatever/freezer.state

Read more at https://man7.org/linux/man-pages/man7/cgroups.7.html and https://www.kernel.org/doc/Documentation/cgroup-v1/freezer-subsystem.txt.

Now that the process’s tasks tree is all frozen, CRIU needs to collect all relevant tasks’ resources and write them to dump files, which should later be used for restore.

The first set of resources being dumped are collected simply by reading procfs. These resources are VMAs (Virtual Memory Areas), memory-mapped files, and opened file descriptors. In addition, registers and other core task parameters are collected using ptrace and parsing /proc/$pid/stat.

Afterwards, CRIU injects a parasite code into each task’s address space whose job is to collect some more information such as credentials and actual memory contents.

More information about how the parasite code injection is done can be found here.

The final step in the checkpointing process is cleaning up CRIU’s parasite code, restoring original code, detaching ptrace, and then resuming tasks from where they’ve been stopped (actually this is optional and can be turned-off by specifying -R|--leave-running).


The restore process is pretty straightforward. During restore CRIU gradually morphs itself into the target process.

First off, the restorer process reads all dumped image files and finds out which processes share which resources. Next, it re-creates all processes in the tree by calling fork(). Note that threads are not restored here but rather on the last stage.

The PID Dance

Each forked process is supposed to be assigned with its original pid. But how exactly?

Well, in order to do so, CRIU utilizes a feature introduced in Kernel v3.3 used by the kernel for keeping track of the last pid it has assigned.

It is accessible through the sysctl file /proc/sys/kernel/ns_last_pid and is basically an incrementing counter of process ids. It requires CONFIG_CHECKPOINT_RESTORE to be set and it is enabled by default in most of Linux distributions.

So in order to fork a process with a desired pid, say 3214, CRIU does the following:

1. Opens /proc/sys/kernel/ns_last_pid
2. Locks file
3. Sets ns_last_pid's new value to pid-1 (3214 - 1)
4. Closes /proc/sys/kernel/ns_last_pid
5. Clones process so that the child process is supposed to have pid 3124
6. Calls getpid() inside the child process to validate desired pid
7. Unlocks file
8. Voilà

Very simple and yet cool!

Bear in mind that if any pid already exists when trying to restore, then the restore will fail. The solution is to restore the process inside a different pid namespace (and mount namespace — see https://criu.org/CR_in_namespace).

There are a few caveats to this approach, the main ones being it too slow due to multiple syscalls required for each such clone, and that it is open to race conditions.

As explained in https://lisas.de/~adrian/criu-and-the-pid-dance-article.pdf:

It can always happen that between setting the desired PID via ns_last_pid and the actual clone() another process, independent of the restore, is created, which means that getpid() will not return the desired PID and CRIU will abort.

Enter clone3() and set_tid.

Using Kernel v5.3’s clone3() (note there’s no matching glibc wrapper yet), and set_tid array available from v5.5, we can now explicitly select specific process ids for a cloned process, in some or all of the PID namespaces where it is present, directly when we call clone3(). This essentially eliminates the race conditions and saves us from multiple syscalls requirement of the ns_last_pid method. It is currently supported by CRIU.

Now that all of the required processes were created, CRIU will carry on and restore resources such as opening file descriptors, preparing namespaces, opening anonymous shared mappings, opening file mappings, opening & pre-mapping private memory areas, opening sockets, and more…

For the final step, CRIU will switch to the restorer context — cleaning up its own memory mappings — and restore all other resources left: threads, timers (so they will fire as late as possible), credentials & security settings (so they won’t limit us during the restore process), private memory areas re-mappings (using mremap), file mappings (using mmap), and anonymous shared mappings (also using mmap).

From now on, the process is restored and will continue to run from where it was originally checkpointed.

If you’d like to dive even deeper inside the internals, have a look at https://criu.org/Category:Under_the_hood :)

Live Migration

Live migration is the process of moving a running Virtual Machine or Application between two different nodes while keeping clients connected. Memory, relevant storage and network connectivity should all be transfered.

CRIU is a perfect match for these kind of tasks, and is actually used in production by some big companies. For instance, Google uses CRIU for live migrating containers inside its Borg clusters.

Lets use CRIU ourselves to demonstrate the migration of a simple loop script from our local machine to a virtual machine.

Our script looks like this (test.sh):

while :; do
    sleep 1

In order to checkpoint the script’s process we run:

sudo criu dump -t <pid> --images-dir ~/demo/images --shell-job && echo OK
  • --images-dir indicates where to dump the image files
  • --shell-job tells CRIU that our process was spawned from a shell

We then scp (i.e, transfer) both the script (so CRIU is able to restore its fd) and the dumped images to our VM, and finally restore the process using:

 sudo criu restore -D <path-to-images-dir> -vvv --shell-job -d
  • -vvv for higher verbosity
  • --shell-job again to let CRIU known it was spawned from a shell
  • -d so that the restored process will run in background

And that’s it!

Some more cool CRIU tutorials can be found here.